The present disclosure relates to an avalanche photodiode having edge breakdown suppression. More particularly, the disclosure relates to an avalanche photodiode which is configured so as to undergo a predetermined sequence of depletion which suppresses edge breakdown.
The internal gain provided by avalanche photodiodes (APDs) allows for the reduction or elimination of more noisy external amplifiers in optical detection systems. For this reason, avalanche photodiodes are attractive for use in detection applications that require optical gain. Such applications include, for instance, use in telecommunications networks, range finders, biomedical imagers, and scintillation detectors.
As is known in the art, planar avalanche photodiodes typically are easier to passivate than mesa avalanche photodiodes in that planar avalanche photodiodes do not need to be etched during the manufacturing process as do mesa avalanche photodiodes. This translates into improved reliability for planar avalanche photodiodes. For this reason, it is generally regarded as desirable to use planar avalanche photodiodes whenever possible. Despite their advantages over mesa avalanche photodiodes, planar avalanche photodiodes often suffer from edge breakdown. Edge breakdown refers to the phenomenon in which the carrier generation rate at the edge of the avalanche photodiode is substantially larger that that of the center region of the avalanche photodiode. This phenomenon is graphically illustrated in FIG. 1 which shows the carrier generation rate over a radial section of a conventional avalanche photodiode. As indicated in this figure, a spike in the graph occurs adjacent the edge of the device, indicating an edge breakdown condition. With this concentration of carrier generation occurring at the edge of the avalanche photodiode, a relatively high current exists in a relatively small region. This condition suppresses the amount of gain that can be obtained from the device. In addition, this condition further negatively affects device reliability, illuminated and dark current-voltage (I-V) characteristics, leakage current, noise, and yield.
In view of the above, the location of avalanche breakdown is of major concern in the design of planar avalanche photodiodes. Of paramount importance to avoiding edge breakdown (and achieving center breakdown) is the electric field distribution within the avalanche photodiode because impact ionization has an exponential dependence on electric field. The field dependence of the ionization coefficient xcex1n,p, is given by                               α                      n            ,            p                          =                              α                          n              ,              p                        ∞                    ·                      exp            ⁡                          (                              -                                                      (                                                                  E                                                  n                          ,                          p                                                crit                                                                    E                        *                                                              )                                                        β                                          n                      ,                      p                                                                                  )                                                          [                  Equation          ⁢                      xe2x80x83                    ⁢          1                ]                                          E          *                =                                            "LeftBracketingBar"                                                E                  →                                ·                                                      J                    →                                                        n                    ,                    p                                                              "RightBracketingBar"                                      "LeftBracketingBar"                                                J                  →                                                  n                  ,                  p                                            "RightBracketingBar"                                .                                    [                  Equation          ⁢                      xe2x80x83                    ⁢          2                ]            
These equations show that the generation rate due to impact ionization is also exponentially dependent on the flux of the carriers entering the high field regions in [1] and [2], where {right arrow over (J)}n and {right arrow over (J)}p are the electron and hole current densities, {right arrow over (E)} is the electric field, and E * is the electric field in the direction of the current density. The variables xcex2n, xcex2p, xcex1n∞, xcex1p∞, Encrit, and Epcrit are experimentally determined parameters that characterize the ionization coefficients.
In order to improve device reliability and reduce leakage currents, avalanche photodiodes must be designed so the impact ionization in the center region of the avalanche photodiode dominates the breakdown process. Currently, there are three primary techniques used for suppressing edge breakdown in planar avalanche photodiodes. The first technique relies on altering the geometry at the edge of the well structure in such a way as to reduce the electric field strength in that region. Examples of this technique include the use of doubly diffused wells, pre-etched wells, and guard rings. Of these three examples, by far the most common approach is the use of guard rings. As known in the art, guard rings are rings of material having a different dopant level which are formed around the periphery of the avalanche photodiode. When a guard ring is correctly formed, its presence changes the electric field profile of the avalanche photodiode to suppress edge breakdown. Unfortunately, the desired electric field profile is often difficult to obtain in that guard ring designs are sensitive to the depth of implantation and the dose of the implant. In terms of implantation depth, if the implant is too deep, tunneling will occur in the absorption layer. If, on the other hand, the implant is too shallow, then edge breakdown will occur. Similarly, incorrect dopant levels can prevent the guard ring from effectively suppressing edge breakdown.
The second known edge suppression technique relies on redistribution of the equipotential regions near the well edge. Single and double floating guard rings are examples of this method. Floating guard ring designs typically require at least two wells (primary and floating) to achieve center breakdown. In that the location of breakdown is highly dependant upon the lateral position of the floating guard ring, edge breakdown may still occur at the edge of the primary well if the guard ring is fabricated too close to the primary well.
The third edge suppression technique confines the highest electric field to the center of the well away from the edges by introducing a charged disc underneath the well. Two tiered charge sheets are representative of this approach. The two-tiered charge sheet suppression technique is advantageous since it only requires surface processing related to the formation of the primary well. However, additional complexities arise in the fabrication of the mesa charge disc. For instance, the avalanche photodiode must be removed from the growth chamber and then etched to form the mesa. This makes it a two-growth process which requires realignment of the device for the second growth step. As known in the art, the interruption of the growth process increases the difficulty of obtaining good yield, high reliability, and proper leakage current.
Conventional edge breakdown suppression techniques can also negatively impact device capacitance. The capacitance in planar avalanche photodiodes is proportional to the width of the well and is inversely proportional to the depth of the depletion beneath the well. Most edge breakdown suppression mechanisms rely on altering the geometry of the primary well. This is the case with guard ring and dual diffusion designs. Altering the geometry of the well edge results in increase in the effective area of the well. This, in turn, results in higher device capacitance. Although avalanche photodiodes with buried charge sheets do not alter the primary well geometry, the primary well must be wider than the width of the buried charge sheets. This increases the well area needed for a given active region width and subsequently also increases the device capacitance.
In addition to capacitance, conventional edge breakdown suppression techniques generally increase the dark current. This occurs with the guard ring and buried charge sheet techniques because these techniques deplete a larger volume of the absorption region than is needed for photon detection. Since the absorption region is generally made from a material with a smaller bandgap than the multiplication region, the majority of the dark current comes from the depleted absorption region.
From the foregoing, it can be appreciated that it would be desirable to have an avalanche photodiode which effectively suppresses edge breakdown and which does not suffer from the disadvantages highlighted in the foregoing.
The present disclosure relates to an avalanche photodiode having edge breakdown suppression. The photodiode comprises a p contact and an n contact, as well as a plurality of device layers disposed between the p contact and the n contact. The device layers include, in order from the p contact to the n contact, a primary well, a decoupler layer, a multiplication layer, a charge sheet, an absorption layer, and a substrate.
The layers are constructed so as to have particular volumes of charge which affects the order in which they deplete. For instance, the multiplication layer has a volume of charge less than that of the decoupler layer, and the decoupler layer has a volume of charge less than that of the charge sheet. With this construction, the multiplication layer will deplete before the decoupler layer and the decoupler layer will deplete before the charge sheet when a negative bias is applied to the avalanche photodiode. This results in a joint opening effect within the avalanche photodiode which effectively suppresses edge breakdown.
The present disclosure further relates to a method for suppressing edge breakdown in an avalanche photodiode which comprises applying a negative bias to the avalanche photodiode at its p contact, increasing the bias such that a multiplication layer of the avalanche photodiode is first depleted, further increasing the bias such that a decoupler layer of the avalanche photodiode is depleted, and further increasing the bias such that a center region of a charge sheet of the avalanche photodiode is depleted. This sequence of depletion creates a joint opening effect which suppresses edge breakdown.
The features and advantages of the invention will become apparent upon reading the following specification, when taken in conjunction with the accompanying drawings.